We are proposing to continue a multi-year conference grant that has provided a forum for researchers to pursue collaborative studies of the molecular genetics of attention deficit hyperactivity disorder (ADHD). The original application was conceived in response to a call from the NIMH for researchers to establish mechanisms for collaborating in a manner that would facilitate the detection of genes predisposing to psychiatric disorders. ADHD is a common disorder of childhood associated with school failure, psychiatric comorbidity and psychosocial disability. Because family and twin studies suggest that ADHD has a substantial genetic component, several research groups have been pursuing molecular genetic studies of the disorder. These studies have already produced several meta-analytic findings implicating the DRD4, DAT1, DRD5, SNAP-25, and 5HT1B genes in the etiology of ADHD. Although these findings are intriguing, they have not led to new pathways for treatment. Because genomewide linkage studies have been equivocal and a genomewide association scan of 938 ADHD trios found no genomewide significant associations, susceptibility genes for ADHD must, individually, have very small effects. Thus, discovering replicated associations for ADHD will require large samples and collaborative efforts. Collaborative strategies have been successful for diabetes, Crohn's disease and other complex disorders, but they required very large samples. Despite the need for collaboration, collaboration can be difficult. Many investigators are concerned that large collaborative studies will dilute the scientific impact of their work and will make it difficult for junior investigators to establish independent reputations. Moreover, when collaborations are considered, they frequently face hurdles that cannot be surmounted. For example, clinical traditions at each site often clash regarding what diagnostic instruments are appropriate for use. This leads to the creation of data sets that are not easily combined with one another. Although we have begun to work out many of these issues at prior conferences, we need to continue the series to build upon those achievements and to complete the collaborative plans laid out in prior conferences.